How Much Strength Does a Plant Need to Take Up Water?

The answer lies in tension or water potential (Ψ), and understanding it is key to efficient irrigation.

Water travels from the soil to the atmosphere through a tension gradient:

🔻 Soil Ψ ≈ -0.3 bars
🔻 Root Ψ ≈ -1 bar
🔻 Xylem Ψ ≈ -3 bars
🔻 Leaf Ψ ≈ -7 bars
🔻 Atmosphere Ψ ≈ -1000 bars

💧 This gradient is driven by atmospheric demand, creating a chain of negative tension from the stomata to the root. It is at this point where tensions between the root and soil may come into conflict:

If soil Ψ is lower than root Ψ, the plant cannot take up water, which can happen for several reasons (here are 3 examples):

1️⃣ The plant closes stomata due to high atmospheric water demand (e.g., high temperatures or high vapor pressure deficit). It can also happen that atmospheric demand is nearly zero and little tension is generated.

2️⃣ Insufficient or unbalanced root system relative to leaf volume (e.g., physical soil limitations or lack of stimuli for root growth).

3️⃣ Lack of water in the soil (e.g., due to poor irrigation management or design, or making decisions without monitoring the soil).

For the latter, the negative tension generated by soil depletion depends on the soil type, so depleting 30% of soil water content will have different effects depending on the characteristics of the soil and crop.